minor changes

ld3/Sample from mathworks/test_dds/test_dds.m

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+clc;clear all;close all;
+% DDS (Direct digital synthesizer) 
+% https://en.wikipedia.org/wiki/Direct_digital_synthesizer
+
+% Direct digital synthesizer (DDS) is a type of frequency synthesizer
+% used for creating arbitrary waveforms from a single,
+% fixed-frequency reference clock.
+% Applications of DDS include: signal generation, local oscillators
+% in communication systems, function generators, mixers, modulators,
+% sound synthesizers and as part of a digital phase-locked loop
+
+
+sintablen = 22050;
+SINTAB = sin(2*pi*(0:sintablen-1)./sintablen);
+fs = 22050;
+
+% the sintable consists of one cycle of sine wave with 2048 samples
+% if you access 2048 samples/sec (fs) from the above sintable,
+% it will generate one complete cycle of sine wave in one sec.
+% so, effectively the frequency is 1Hz.
+% step = 1; Feff(Effective frequency) = fs/sintablen;
+F_required = 1;
+index = 1; step = (F_required/fs)*sintablen;
+for i = 1:2048
+    sin1Hz(i) = SINTAB(round(index));
+    index = index+step;
+    if index>sintablen
+        index = index-sintablen;
+    end
+end
+plot(sin1Hz); hold on;
+
+% suppose we need to generate 2Hz sine wave.
+% step = 2;
+F_required = 1664;
+index = 1; step = (F_required/fs)*sintablen;
+for i = 1:2048
+    sin2Hz(i) = SINTAB(round(index));
+    index = index+step;
+    if index>sintablen
+        index = index-sintablen;
+    end
+end
+plot(sin2Hz,'r'); hold on;
+
+% suppose we need to generate 0.75Hz sine wave
+
+index = 1;
+F_required = 2496;
+step = (F_required/fs)*sintablen;
+for i = 1:2048
+    sinp75Hz(i) = SINTAB(round(index));
+    index = index+step;
+    if index>sintablen
+        index = index-sintablen;
+    end
+end
+plot(sinp75Hz,'k'); hold on;
+legend('1Hz','2Hz','0.75Hz')
+
+% M Manoj Kumar
+% could be reached at m.mnjkmr@gmail.com